Field of Invention
The disclosure relates to optical fibers. More particularly, the disclosure relates to an apparatus and a method of recovering optical power degradation of rare-earth-doped optical fibers under irradiation.
Description of Related Art
Space technology plays an increasingly important role in our daily life. However, spacecraft is typically hard or expensive to reach for service after launch. Therefore, components, sub-assemblies and systems for space applications require special design and rigorous test in order to perform over space environment, especially irradiation.
Due to the advantages of light weight, compact size, broad bandwidth, and resistance of electromagnetic interference, fiber optics systems and assemblies have been developed and employed for space and nuclear facilities applications including communication, sensing, navigations and etc. However, it has been known that the performance of an optical-fiber-based device would be severely degraded by the RIA (Radiation-Induced-Attenuation). The RIA could cause severe output power loss of the optical-fiber-based device, and hence greatly limits its operating life.
Some methods, such as thermal annealing, hydrogen pre-loading, and photo-annealing, have been reported to reduce the RIA effect onto optical fibers. For thermal annealing, it typically requires to take up to 300° C. to decrease RIA effectively. Such a high temperature environment could damage the device itself, and therefore could not be employed practically.
For hydrogen pre-loading method, it needs a hermetic coating to avoid out-diffusion of hydrogen, and the fabrication process of a hermetic coating might be complicated and add additional cost, and could limit flexibility in tight bending.
The photo-annealing of irradiated fibers has been studied for many years. The fibers used in previous studies are pure core fibers and Ge-doped fibers. It has been known that some Si and Ge related defects can efficiently be photo-annealed. However, the RIA of an Al-doped EDF (erbium-doped fiber) is dominated by Al related defects, not Si or Ge related defects. The effect of RIA recovery in Al-doped EDF by photo-annealing method was reported by Peng and et al [U.S. Pat. No. 9,077,143, and U.S. Pat. No. 8,578,739]. However, those two patents did not cover the employment of the unique laser mode switching technique as disclosed in the present invention to achieve pumping and photo-annealed effects sequentially onto a rare-earth-doped fiber assembly under irradiation.